1. Field of the Invention
The present invention relates to froth flotation apparatuses and, in particular, to froth flotation apparatuses for the deinking of waste paper.
2. Prior Art
The process of froth flotation has many applications, including the separation of minerals from one another, the recovery of oil from oil sands, the removal of particulate contaminants from a liquid, and the flotation of solids from white paper in paper making. Another well known application of the floth flotation process is the deinking of waste paper.
One type of deinking apparatus, known as a deinking cell, includes a cylindrical vessel defining the cell which includes an upstanding reject pipe concentrically mounted within the vessel, a gray stock inlet conduit, and a deinked stock accept outlet conduit which communicates with the bottom of the vessel. A foam removing device, which may consist of a stationary doctor blade, is attached to an inside surface of the vessel wall and extends across the cell at an appropriate height to the mouth of the reject pipe. A blower is mounted to blow air across the doctor blade toward the reject pipe. The inlet conduit includes an aerating apparatus or sparger which creates a multiplicity of air bubbles that attract ink particles, thereby forming a froth or foam which floats upwardly and collects on the surface of the fluid within the cell. The inlet conduit is positioned with respect to the cell such that fluid flowing into the cell through the conduit flows in a vortical pattern about the interior of the vessel, thereby eliminating the need for additional mixing apparatus inside the vessel.
The foam collecting on the surface of the fluid flows around the interior of the vessel with the fluid and is brought into contact with the doctor blade where it collects in a pile. The blade is angled so that the collecting foam is directed along the blade by the blower to the reject pipe and is removed from the cell. The deinked stock flows from the cell through the outlet conduit.
The sparger or mixing chamber consists of a conically shaped portion of the inlet conduit within which is mounted a conical nozzle having perforate walls. The shape of the nozzle matches the shape of the conical conduit section, and the nozzle is adjustably displaceable within the section to bring the perforate walls toward or away from the conical section. The fluid bearing the gray stock is brought around and past the conical nozzle, which communicates with a source of compressed air. The compressed air flows through the nozzle and enters the fluid stream in the form of a multiplicity of fine bubbles. By displacing the nozzle toward the conical walls of the conduit, the overall cross-sectional area of the space between the nozzle and conduit section is descreased, thereby increasing the relative velocity of the fluid with respect to the surface of the nozzle. This increase in fluid velocity causes the bubble size to diminish. The smaller bubbles are able to collect and adhere to ink particles of a sort which cannot be collected by larger bubbles, thereby increasing the efficiency of ink removal.
There are several disadvantages with this type of cell. For example, additional machinery is required to displace the aerating nozzle within the conical conduit section of the sparger, which adds to the overall cost of the apparatus. Another disadvantage of the sparger arises when the nozzle is displaced within the conical conduit section to increase the fluid flow velocity. At this time, the effective area of the conduit is descreased and frictional forces of the fluid against the walls of the conduit and the walls of the aerator are increased, requiring an increased pressure head and necessitating the use of larger-sized pumps, also adding to the overall cost of the apparatus.
The doctor blade of this device also possesses disadvantages. For example, the blade cannot be adjusted to compensate for variations in the fluid level within the cell, so that the fluid level must be maintained within a predetermined range. If the fluid level rises above the level of the blade, the foam is not removed from the surface of the fluid and guided to the reject pipe. If the fluid level drops below the level of the blade, the efficiency of the operation is decreased since foam may pass beneath the blade.
Furthermore, the doctor blade may become coated with foam which would necessitate periodic cleaning. In addition, the blade acts to smooth the surface of the fluid within the vessel, thereby increasing the likelihood of flocculation of pulp on the surface of the fluid.
It is also known to arrange a plurality of the aforementioned cells in series in a stacked relation, known as a tower. For example, U.S. Pat. No. 4,186,094 discloses a tower in which the lowermost cell functions as the primary cell and receives gray stock from the source to perform a first froth flotation process. The outlet conduit which conveys the partially deinked pulp is joined to the cell immediately above it where it is subjected to a second froth flotation process, and so on. A disadvantage with such towers is that the placement of the primary cell as the lowermost cell requires additional mechanical pumps to lift the effluent to higher cells, adding to the cost of the system.
In applications other than froth flotation processes, it is known to utilize a skimming apparatus which is buoyant so that it floats upon the surface of the fluid skimmed. For example, in U.S. Pat. Nos. 4,054,520 and 4,166,036, there is disclosed skimming apparatus for removal of scum or other contaminated particles from the surface of a liquid contained in a tank. In both applications, the skimming apparatus is displaced across the surface of the fluid within the tank, causing the material floating on the surface of the fluid to be pushed to one side where it flows to a trough or other removal apparatus. However, such devices are disclosed as used with tanks of a rectangular shape, and would be unsuitable for the cylindrical tanks of a froth flotation apparatus. Furthermore, the complexity of such displaceable floating skimmers would greatly add to the overall cost of the system.
Accordingly, there is a need for a froth flotation apparatus in which the size of the bubbles forming the foam can be varied to maintain a predetermined size so that a particular type of ink may be removed. There is also a need for a froth flotation apparatus having foam removing apparatus which is automatically raised or lowered as the fluid level within the cell raises or lowers, which does not increase flocculation within the cell, or which requires frequent cleaning. Furthermore, there is a need for a froth flotation system of a plurality of cells in series which does not require mechanical pumping means between cells to maintain a sufficient pressure head to perform the flotation process for each cell.